Abstract. Although afterschool programs (ASPs) have been recognized as a critical context for promoting youth physical activity (PA), with broad reach to underserved communities, research by the PI and others have indicated that ASPs continue to struggle to meet policy guidelines for daily youth PA. Moreover, despite continued increases in obesity rates among U.S. adolescents, there have been few ASP interventions focused on underserved adolescents, and limited-to-no impact on adolescents' sustained behavior change. Previous ASP studies by the PI demonstrate that social mechanisms overlooked in previous interventions (e.g., developing friendship and connection to peers and staff through PA; group belonging, including positive peer PA norms and tangible support) are key predictors of youth PA. The research team recently completed a feasibility trial (NIH R21 HD077357) within 6 ASPs (3 intervention vs. 3 standard- health curriculum controls) for underserved middle school youth that was the first intervention to date to address these youth PA social goals/mechanisms. Findings showed positive effects for changing staff behaviors, improving youth PA social supports and connections with peers and staff, and increasing youth PA. The proposed study will expand on the previous ASP feasibility trial with a novel translational approach that works with pre-existing ASPs and targets staff as instrumental for sustainable changes in social mechanisms within the ASP setting for increasing the daily PA of underserved adolescents. The proposed project will be a prospective, randomized controlled trial comparing a staff-based social development PA program to a standard ASP health curriculum control. The proposed efficacy trial will enhance the influence of pre-existing ASP staff as key change agents and address social developmental needs of adolescents overlooked in previous interventions through employing a novel theoretical framework that expands on social developmental theory and the social mechanisms highlighted by Self-Determination Theory, Achievement Goal Theory, and Social Cognitive Theory. The intervention will target the three key social mechanisms of ?PA social affiliation goals? for increased and sustained PA of staff and underserved middle school youth (i.e., Friendships/connections through PA; Group Belonging, and; Staff Connection). All components of the intervention (health promotion initiative, comprehensive training, and tailored social PA curriculum) aim to improve staff capacity for facilitating a PA context that supports these social goals/mechanisms (connections, group belonging) and increases the influence of ASP staff as positive PA role models and agents of change. Compared to control ASPs, ASPs receiving the `Connect' program are expected to show greater improvements from baseline to post- and 6 mo follow-up in social mechanisms, youth PA, and staff PA. The results of this proposed project will demonstrate the efficacy of the intervention, and will assist in developing a model of training, motivating, and empowering ASP staff to address social mechanisms that promote youth PA for conducting a large scale effectiveness trial.